This proposal deals with the delineation of a new mechanism of antibody independent complement lysis of lymphoblastoid cells as an in vitro model for in vivo tumor rejection mechanisms. A unique feature of this mechanism lies in the participation of the complement receptors (C3b, C3d, C4b receptors). To assess the frequency of occurrence of this mechanism a variety of receptor bearing lymphoblastoid cells will be tested for susceptibility to lysis by complement activated by cobra venom factor, receptor bound C3b or C4b,2a. On synchronized cultures we will assess an influence of the cell growth cycle on receptor expression and susceptibility to lysis. We will study the physico-chemical and functional properties of receptor bound complement proteins by assessing whether bound C3 and C4 can be cleaved by complement enzymes, other serum enzymes and trypsin, and whether C3b-inactivator or trypsin effect bound C3b and C4b. The radiolabeled complement proteins will be eluted from cells and analyzed on immunoelectrophoresis and SDS polyacrylamide gels. Using radiolabeled components, the formation of hemolytically active C3b, C3A and C4b,2a complexes on receptor bound C3b and C4b will be examined. We will further investigate whether such complexes formed in fluid phase can bind to receptors and induce cell lysis. On cells resistant to lysis membrane associated complement activation will be assessed with fluorescein labeled antibodies detecting neoantigens on the cell bound C5b-9 complex. We will attempt to establish evidence for the existence of this mechanism in vivo by analyzing peripheral blood cells and tumor cells of patients with hemopoietic malignancies for the presence of the C3b, C3A and C4b,2a complexes. Complexes on membranes will be demonstrated by capping experiments using fluorescent antibody techniques.